Method for making silicon carbide whiskers

ABSTRACT

A ceramic composite made by compacting a starting powder blend. The composite includes between about 50 volume percent and about 99 volume percent of a ceramic matrix; and between about 1 volume percent and about 50 volume percent as-processed silicon carbide whiskers. The ceramic composite having a fracture toughness (K IC ) of greater than about 4.0 MPam 1/2 . The ceramic has a silicon carbide whisker density as measured in whiskers per square millimeter equal to or less than about 1500 times the volume percent of silicon carbide whiskers, but in a density sufficient for the ceramic composite to have the fracture toughness.

FIELD OF THE INVENTION

The present invention pertains to a ceramic body, and a method of makingthe same, that has whisker reinforcement. More specifically, theinvention pertains to a ceramic cutting tool (coated or uncoated), and amethod for making the same, that has as-processed coarse silicon carbidewhisker reinforcement wherein the ceramic matrix comprises any one ormore of alumina, a carbide, nitride and/or carbonitride of titanium,hafnium, molybdenum, zirconium, tantalum, niobium, vanadium and/ortungsten. The invention also pertains to the a processed coarse siliconcarbide whiskers themselves, as well as a method of processingas-received coarse silicon carbide whiskers to produce the as-processedcoarse silicon carbide whiskers.

BACKGROUND OF THE INVENTION

In the past, there have been ceramic matrices (e.g. alumina, boroncarbide, and mullite) with silicon carbide whisker reinforcement such asthat disclosed in U.S. Pat. No. 4,543,345 to Wei (U.S. Reissue Pat. Nos.32,843 [reissued on Jan. 24, 1989] and 34,446 [reissued on Nov. 16,1993] to Wei). According to the Wei patent, the incorporation of siliconcarbide whiskers increased the fracture toughness of the ceramic body.The Wei patent mentions two specific kinds of silicon carbide whiskers;namely, the grade F9 (or SC-9) silicon carbide whiskers from ARCO (nowAdvanced Composite Materials Corporation of Greer, S.C.) and “Tokamax”silicon carbide whiskers from Tokai Carbon Company, Tokyo, Japan. Theaverage diameter of these whiskers was 0.6 micrometers with a length of10-80 micrometers and an average aspect ratio of 75.

There have also been ceramic cutting tools with silicon carbide whiskerreinforcement. In this regard, U.S. Pat. No. 4,789,277 to Rhodes et al.entitled METHOD OF CUTTING USING SILICON CARBIDE WHISKER REINFORCEDCERAMIC CUTTING TOOLS and U.S. Pat. No. 4,961,757 to Rhodes et al. forREINFORCED CERAMIC CUTTING TOOLS each disclose the use of siliconcarbide whiskers (the content ranges from 2 volume percent to 40 volumepercent) alumina matrix. The alumina matrix may be “doped” with up to30% zirconia, hafnia and titanium carbide. The particular siliconcarbide whiskers disclosed in these Rhodes et al. patents are siliconcarbide whiskers made by the Advanced Materials group of ARCO ChemicalCompany (now Advanced Composite Materials Corporation of Greer, S.C.).These silicon carbide whiskers have an average diameter of about 0.6micrometers and an aspect ratio on the order of 15-150.

PCT/US 86/00528 Patent Application to Rhodes et al. entitled HIGHDENSITY REINFORCED CERAMIC BODIES AND METHOD OF MAKING SAME has as itsfocus the pressureless sintering of whisker-reinforced ceramic bodies.This document mentions a whisker content of between 0.5 and 21 volumepercent. The specific examples teach the use of an alumina matrix withSiC whisker contents from 6.1 volume percent to 29.2 volume percent.This document mentions that the silicon carbide whiskers have lengthsequal to 10-100 micrometers and average diameters on the order of 1.0micrometer or less. The example shows SiC whiskers with a length of10-80 micrometers and a diameter equal to 0.6 micrometers.

U.S. Pat. No. 5,656,217 to Rogers et al. discloses pressurelesssintering of ceramics reinforced with silicon carbide whiskers. Theas-received silicon carbide whiskers are monocrystalline and have adiameter between about 0.4 to about 0.6 micrometers and an aspect ratioequal to 15-150. U.S. Pat. No. 5,389,586 to Rogers et al. entitledPRESSURELESS SINTERING OF WHISKER REINFORCED COMPOSITES suggests usingthe silicon carbide whiskers disclosed in U.S. Pat. No. 4,961,757 toRhodes et al.

U.S. Pat. No. 5,059,564 to Mehrotra et al. for an ALUMINA-TITANIUMCARBIDE-SILICON CARBIDE COMPOSITION pertains to an alumina-based matrixcontaining a dispersion of SiC whiskers and a TiC phase. The SiCwhiskers comprise 1.0 to less than 30 volume percent with the mostpreferred range being 2.5 to 20 volume percent. The silicon carbidewhiskers have a diameter equal to 0.3 to 0.7 micrometers and a length of20 to 50 micrometers. The TiC comprises 5 to 40 volume percent, andpreferably, with up to 3 volume percent of a sintering aid residue.

U.S. Pat. No. 5,439,854 to Suzuki et al. pertains to a cutting tool thatcontains 40 weight percent or more of TiC, and 5 to 40 weight percent ofsilicon carbide whiskers (of a length equal to 1-20 micrometers and adiameter equal to 0.2-1.5 micrometers. The cutting tool may also containup to 40 weight percent alumina, as well as sintering aids. Up to 40weight percent of the TiC may be substituted by Ti or a Ti-basedcompound such as a nitride, boride, or oxide.

U.S. Pat. No. 5,955,390 to Mehrotra et al. (and U.S. Pat. No. 6,204,213to Mehrotra et al.) pertains to a ceramic composite that comprises amatrix and ceramic whisker reinforcement. The examples include atitanium carbide-alumina matrix with silicon carbide whiskerreinforcement, a titanium carbonitride-alumina matrix with siliconcarbide whisker reinforcement, and a titanium molybdenum carbide-aluminamatrix with silicon carbide whiskers. This patent discloses two kinds ofsilicon carbide whiskers. The first are Tokai silicon carbide whiskersthat have an average length equal to 20-50 micrometers with an averagediameter equal to 0.3 to 1 micrometers. The second are the SC-9 siliconcarbide whiskers that had an average length equal to 10-80 micrometersand an average diameter equal to 0.6 micrometers.

U.S. Pat. No. 5,754,142 to Johnsson et al. pertains to SiC whiskerreinforced alumina cutting tools. The '142 patent describes the whiskersas comprising monocrystals with a diameter of 0.2-10 micrometers and alength of 2.5-100 micrometers with a length to diameter ratio preferablyof 5-30.

U.S. Pat. No. 4,956,316 to Sawyer concerns a silicon carbide whiskerreinforced material. The silicon carbide whiskers used in Sawyer aresingle crystals containing alpha, beta and mixed alpha and beta phasesof silicon carbide. The average diameter of the whiskers is about 0.6-2micrometers and the length about 10-80 micrometers. It is preferred thatthe aspect ratio of the whiskers be less than about 30.

U.S. Pat. No. 4,867,761 to Brandt et al. discloses silicon carbidewhiskers that comprise of monocrystals with a diameter of 0.5-10micrometers and a length of 2.5-100 micrometers characterized thereofthat the length/diameter ratio preferably is 5-10.

In the past, silicon carbide whiskers have been subjected to ballmilling. In this regard, U.S. Pat. No. 5,376,600 to Tiegs teachesextensive ball milling. However, in describing prior processes, the '600patent mentions ball milling for as short a duration as 0.5 hours tolower the size distribution. U.S. Pat. No. 5,449,647 to Brandt teachesthat the aspect ratio can be controlled by ball milling.

In the past, silicon carbide whiskers have been subjected to heattreatments. For example, U.S. Pat. No. 5,017,528 to Tiegs et al.concerns the treatment of silicon carbide whiskers. This patent showsheat treating SiC whiskers in an oxygen sparaging atmosphere.

Table I set forth below presents certain physical properties of someprior art commercial cutting tools.

TABLE I Selected Physical Properties Of Certain Commercial Cutting ToolsCutting Hardness VHN (GPa) K_(IC) (E&C) Tool HRA 18.5 Kg Load MPam · ½WG-300 94.6 19.4 6.5-7.5 HC6 94.6 19.4 5.1 K090 94.8 19.1 4.7Referring to these commercial cutting tools, the WG-300 cutting tool issold by Greenleaf Corporation of Saegertown, Pa. and has a compositionof about 25 volume percent to about 30 volume percent SiC whiskers andthe balance alumina. The HC6 cutting tool is sold by NTK Cutting ToolDivision of NGK Spark Plugs (USA), Inc. of Farmington Hills, Mich., andhas a composition of about 70 weight percent TiC and the balancealumina. The K090 cutting tool is made by Kennametal Inc. of Latrobe,Pa. and has a composition of about 70 volume percent alumina and 30volume percent TiC. Each of these compositions may also contain minoramounts of one or more sintering aids. Although not listed in Table Iabove, commercial grade CC670 is sold by Sandvik Coromant. Grade CC670is another silicon carbide whisker-reinforced alumina cutting tool thathas a composition substantially identical to the composition of WG-300.

As can be seen by a review of the above documents, the predominant typeof silicon carbide whisker used as a reinforcement for ceramicsmatrices, and especially the ceramic matrix in a cutting tool, is afiner diameter monocrystalline silicon carbide whisker that has anaverage diameter of about 0.6 micrometers and an average length of about10 to about 80 micrometers and an average aspect ratio of about 75.These finer diameter silicon carbide whiskers are more expensive thanas-received coarse silicon carbide whiskers (i.e., silicon carbidewhiskers that have a so-called coarse morphology). Heretofore,as-received coarse silicon carbide whiskers have not been satisfactorilyemployed as reinforcement in ceramic matrices, and especially in ceramicmatrices of cutting tools.

Typically, coarse morphology silicon carbide whiskers have an averagediameter of between about 1.2 to about 1.7 micrometers and an averageaspect ratio of between about 6 and about 10. These coarse morphologysilicon carbide whiskers also have a high percentage of silicon carbideclusters (also known as mat) wherein these clusters may comprise up toabout 20 weight percent of the silicon carbide whiskers and have a sizeas high as 50 micrometers. Scanning electron microscopy (SEM) has shownthat this mat is actually silicon carbide whiskers bonded together bysilica. The bond between these silicon carbide whiskers and the silicais relatively strong so that techniques like ultrasonication or the useof chemical dispersants will not break up the mat. These coarsemorphology silicon carbide whiskers also have a rough surface whereinthe surface area (BET) may be greater than about 3 square meters pergram. These coarse morphology silicon carbide whiskers also may have ahigh percentage of free silica wherein the silica comprises about 3 (orpossibly up to about 5) weight percent of the silicon carbide whiskers.

The presence of the silica mat is disadvantageous to the effectivereinforcement of the matrix by the silicon carbide whiskers. The same istrue for the high free silica content. The rough surface (i.e., highsurface area) of the silicon carbide whisker also is disadvantageous tothe effective reinforcement of a ceramic matrix by the as-receivedcoarse silicon carbide whiskers. Ineffective reinforcement may be due toany one or more of low density, reduced hardness or reduced fracturetoughness of the ceramic composite.

It would be desirable to provide a method to process as-received coarsesilicon carbide whiskers so that these whiskers would be suitable foruse as a reinforcement in ceramic matrices. More specifically, it wouldbe desirable to provide a method that processes the as-received coarsesilicon carbide whiskers so as to reduce (or even eliminate) the contentof the mat in the silicon carbide whiskers. It would also be desirableto provide a method that processes the as-received coarse siliconcarbide whiskers that reduces (or even eliminates) the free silicacontent. Finally, it would be desirable to provide a method thatprocesses the as-received coarse silicon carbide whiskers so as toreduce the surface roughness of the silicon carbide whiskers.

SUMMARY OF THE INVENTION

In one form, the invention is a ceramic cutting tool comprising aceramic composite. The composite has a rake face and a flank face thatintersect to form a cutting edge. The composite is made by compacting astarting powder blend. The blend comprises between about 50 volumepercent and about 99 volume percent of a ceramic matrix, and betweenabout 1 volume percent and about 50 volume percent as-processed siliconcarbide whiskers. The ceramic composite has a fracture toughness(K_(IC)) of greater than about 4.0 MPam^(1/2). The ceramic composite hasa silicon carbide whisker density as measured in whiskers per squaremillimeter equal to or less than about 1500 times the volume percent ofsilicon carbide whiskers, but in a density sufficient for the ceramiccomposite to have the fracture toughness.

In yet another form, the invention is a coated ceramic cutting tool thatincludes a substrate. The substrate has a rake face and a flank facethat intersect to form a cutting edge. The substrate comprises a ceramicmatrix wherein the ceramic matrix comprises between about 50 volumepercent and about 99 volume percent of the substrate. The substrateincludes silicon carbon whiskers. The substrate has a fracture toughness(K_(IC)) of between about 4.0 MPam^(1/2) and about 8.0 MPam^(1/2). Thesubstrate has a silicon carbide whisker density as measured in whiskersper square millimeter equal to or less than about 1500 times the volumepercent of silicon carbide whiskers, but in a density sufficient for thesubstrate to have the fracture toughness. A coating is on at least aportion of the substrate.

In another form the invention is a ceramic composite that comprises asubstrate wherein the substrate comprises a ceramic matrix and siliconcarbide whiskers. The ceramic matrix comprises between about 50 volumepercent and about 99 volume percent of the substrate. The siliconcarbide whiskers comprise between about 1 volume percent and about 50volume percent of the substrate. The ceramic substrate has a fracturetoughness (K_(IC)) between about 4.0 MPam^(1/2) and about 8.0MPam^(1/2). The substrate has a silicon carbide whisker density asmeasured in silicon carbide whiskers per square millimeter equal to orless than about 1500 times the volume percent of silicon carbidewhiskers, but in a density sufficient for the substrate to have thefracture toughness.

In still another form, the invention is a method of making a ceramiccomposite comprising the steps of: providing as-processed siliconcarbide whiskers having the following properties: an average length ofbetween about 8 micrometers and about 60 micrometers; an averagediameter of greater than or equal to about 1.2 micrometers; an averageaspect ratio of between about 3 and 50; an oxygen content of less thanabout 2 weight percent of the silicon carbide whiskers; and a surfacearea of between about 1 square meters per gram and about 2 square metersper gram; blending a mixture of the as-processed silicon carbidewhiskers, a sintering aid, and one or more ceramic powders; andcompacting the blended powders into the ceramic composite wherein theceramic composite has a fracture toughness (K_(IC)) of between about 4.0MPam^(1/2) and about 8.0 MPam^(1/2), and the ceramic composite has asilicon carbide whisker density as measured in whiskers per squaremillimeter equal to or less than about 1500 times the volume percent ofsilicon carbide whiskers, but in a density sufficient for the ceramiccomposite to have the fracture toughness.

In still another form, the invention is a method of treating as-receivedsilicon carbide whiskers having the following properties: an oxygencontent of about 3 to about 5 weight percent, a surface area of greaterthan about 3 square meters per gram, comprising the step of: heattreating the as-received silicon carbide whiskers at a temperaturebetween about 1400 degrees Centigrade and about 1900 degrees Centigradefor a duration between about 15 minutes and about 60 minutes to formheat treated silicon carbide whiskers having the following properties:an oxygen content of less than about 2 weight percent of the siliconcarbide whiskers, a surface area below about 2 square meters per gram.

In another form the invention is a lot of silicon carbide whiskershaving the following properties: an average length of between about 10micrometers and about 60 micrometers; an average diameter of greaterthan or equal to 1.2 micrometers; an average aspect ratio of betweenabout 6 and about 50; an oxygen content of less than 2 weight percent ofthe silicon carbide whiskers; and a surface area below about 2 squaremeters per gram.

Finally, in another form the invention is a ceramic cutting tool thatcomprises a ceramic composite having a rake face and a flank face, therake face and the flank face intersect to form a cutting edge. Theceramic composite is made by compacting a starting powder blendcomprising between about 50 volume percent and about 99 volume percentof a ceramic matrix. The ceramic composite has a fracture toughness(K_(IC)) of greater than about 4.0 MPam^(1/2). The ceramic composite hasa silicon carbide whisker density as measured in whiskers per squaremillimeter equal to or less than the value along the line A-B of FIG. 8corresponding to the content of the silicon carbide whiskers, but in adensity sufficient for the ceramic composite to have the fracturetoughness.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings wherein thesedrawings form a part of this patent application:

FIG. 1 is an isometric view of a cutting insert that comprises aspecific embodiment of the cutting tool of the present invention;

FIG. 2 is a photomicrograph of the microstructure of a ceramic compositecomprising about 20 volume percent as-processed coarse silicon carbidewhiskers and the balance titanium carbonitride-alumina wherein thesilicon carbide whiskers were processed according to the presentinvention and the photomicrograph has a 10 micrometer scale;

FIG. 3 is a photomicrograph of the microstructure of a ceramic compositecomprising about 25 volume percent as-processed coarse silicon carbidewhiskers and the balance titanium carbonitride-alumina wherein thesilicon carbide whiskers were processed according to the presentinvention and the photomicrograph has a 10 micrometer scale;

FIG. 4 is a photomicrograph of the microstructure of a ceramic compositecomprising about 30 volume percent as-received coarse silicon carbidewhiskers and the balance titanium carbonitride-alumina wherein thesilicon carbide whiskers were not processed according to the presentinvention and the photomicrograph has a 10 micrometer scale;

FIG. 5 is a photomicrograph of the microstructure of a ceramic compositecomprising about 30 volume percent as-received coarse silicon carbidewhiskers and the balance titanium carbonitride-alumina wherein thesilicon carbide whiskers were processed according to the presentinvention and the photomicrograph has a 10 micrometer scale;

FIG. 6 is a photomicrograph of the microstructure of a prior art ceramiccomposite comprising about 25 volume percent as-received fine diametersilicon carbide whiskers obtained from Advanced Composite MaterialsCorporation of Greer, S.C., and the balance titaniumcarbonitride-alumina and the photomicrograph has a 10 micrometer scale;and

FIG. 7 is a photomicrograph of the microstructure of a prior art ceramiccutting tool principally comprised of fine diameter silicon carbidewhiskers (believed to be obtained from Advanced Composite MaterialsCorporation of Greer, S.C.) and alumina, and sold by GreenleafCorporation of Saegertown, Pa. USA under the designation WG-300; and

FIG. 8 is a graph that shows the relationship between the volume percentof silicon carbide whiskers in the ceramic composite (the horizontalaxis) and the density of the silicon carbide whiskers as measured insilicon carbide whiskers per square millimeter (the vertical axis), andhaving Points A and B shown wherein Point A represents a composite with1 volume percent of silicon carbide whiskers and a density of 1500silicon carbide whiskers per square millimeter and Point B represents acomposite with 50 volume percent silicon carbide whiskers and a densityof 75,000 silicon carbide whiskers per square millimeter.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The invention pertains to a ceramic composite body (e.g., a cutting toolor a wear part) that includes a ceramic matrix and as-processed coarsesilicon carbide whiskers as a reinforcement for the matrix. The ceramicbody may be used as a cutting tool or in various wear applications. Inthe context of a ceramic cutting tool, the combination of the matrix andthe whiskers comprise a ceramic substrate. The substrate may be eithercoated or uncoated. The invention also pertains to a method of makingthe coated or uncoated ceramic body.

The invention also pertains to the as-processed coarse silicon carbidewhiskers themselves. The invention further concerns a method ofprocessing the as-received coarse silicon carbide whiskers to producethe as-processed coarse silicon carbide whiskers.

As earlier mentioned, the ceramic cutting tool substrate may optionallybe coated with a hard material. Examples of such hard materials include(without limitation) alumina, titanium carbide, titanium nitride,titanium Carbonitride, titanium aluminum nitride, cubic boron nitrideand diamond and their combinations. The coating may be applied byvarious techniques.

For example, the coating may be applied by chemical vapor deposition(CVD) [see U.S. Pat. No. 4,801,510 to Mehrotra et al. for ALUMINA COATEDSILICON CARBIDE WHISKER-ALUMINA COMPOSITION] or physical vapordeposition (PVD) [see U.S. Pat. No. 5,264,297 to Jindal et al. forPHYSICAL VAPOR DEPOSITION OF TITANIUM NITRIDE ON A NONCONDUCTIVESUBSTRATE] or a scheme where some layers are applied by PVD and somelayers are applied by CVD (see U.S. Pat. No. 5,232,318 to Santhanam etal. for COATED CUTTING TOOLS). It should be appreciated that thesubstrates that have a high titanium carbide or titanium carbonitridecontent, i.e., at least 25 to 30 volume percent titanium carbide ortitanium carbonitride, are electrically conductive to such an extentthat they are particularly suitable for PVD coating, as well as EDMmachining.

Referring to the ceramic matrix, the matrix typically comprises betweenabout 50 volume percent to about 99 volume percent of the total ceramicsubstrate. More preferably, the ceramic matrix comprises between about65 volume percent and about 85 volume percent of the total ceramicsubstrate. Most preferably, the ceramic matrix comprises between about75 volume percent and about 80 volume percent of the total ceramicsubstrate.

In regard to the materials for the matrix of the substrate, as onealternative, the matrix may comprise a carbide, carbonitride and/ornitride (along with the residue of one or more sintering aids) of one ormore of titanium, hafnium, molybdenum, zirconium, tantalum, niobium,vanadium and/or tungsten so that this component is between about 50volume percent to about 99 volume percent or more of the matrix.

As another alternative, the matrix may comprise only alumina and theresidue of one or more sintering aids.

As still another alternative, the matrix may comprise titaniumcarbonitride and alumina along with the residue of one or more sinteringaids. In this alternative, the titanium carbonitride may comprise atleast greater than 50 volume percent of the matrix so that the matrix istitanium carbonitride-based that has a lesser amount of alumina. Thereare two specific compositions of preferred interest in this alternative.Table II below sets forth the starting components of each one of thesecomponents.

TABLE II Starting Components [in Volume Percent] for TitaniumCarbonitride-Based Alumina Composites with Coarse Silicon CarbideWhisker Reinforcement TiCN:Alumina C:N (by SiC Volumetric Material TiCNmole) Alumina Whiskers Yttria Ratio TK5 40.87 0.5:0.5 39.08 20.00 0.2551:49 TK6 38.12 0.5:0.5 36.63 25.00 0.25 51:49

For all of the specific examples set out above in Table II, the yttria(Y₂O₃) was supplied by Hermann C. Strack Berlin GmbH & Co, KG, P O Box1229, D-7887 Lauterburg, Baden, Germany. The Al₂O₃ powder was suppliedby Ceralox (a division of Vista Chemical Company) under the designationHPA-0.5. The Ceralox powder had a BET specific surface area of 10.0 to11.5 square meters/gram. The as-received Ceralox Al₂O₃ contained anaddition of 0.05 volume percent MgO. The Ceralox Al₂O₃ powder wassubstantially equiaxed. The titanium carbonitride powders were suppliedby H.C. Starck Inc., 45 Industrial Place, Newton, Mass. 02161. Thiscomponent has a formula of TiC_(0.5)N_(0.5) and had an average particlesize (FSSS) of 3.0-5.0 μm.

As still another alternative, any one of the above ceramic matrices mayfurther include particulates dispersed therein. These particulates mayinclude aluminum nitride, silicon nitride, titanium boride, zirconiumboride, chromium boride, hafnium boride, alumina, zirconium oxide, andhafnium oxide. The content of these particulates may range between about0.1 volume percent to about 49 volume percent of the matrix. Morepreferably, the content of these particulates may range between about0.25 volume percent and about 25 volume percent of the matrix.

As mentioned above, the matrix includes sintering aid residues. Thepreferred content of the sintering aids is less than or equal to about 1weight percent of the starting powders. A more preferable sintering aidcontent is less than or equal to about 0.5 weight percent of thestarting powders. The most preferable sintering aid content is less thanor equal to 0.25 weight percent of the starting powders. The preferredsintering aids include yttria, magnesia and zirconia either alone or incombination. The sintering aids may also include aluminum nitride andsilicon nitride.

In regard to the ceramic whiskers used as reinforcement in the matrix,these whiskers are as-processed coarse silicon carbide whiskers. It isnot unusual that the silicon carbide whiskers will include up to 20volume percent silicon carbide particulates. The as-processed coarsesilicon carbide whiskers comprise between about 1 volume percent andabout 50 volume percent of the total ceramic composition.

These as-processed coarse silicon carbide whiskers are as-receivedcoarse silicon carbide whiskers that have been processed according tothe process of the present invention to produce the as-processed coarsesilicon carbide whiskers. It is typical that as-received coarse siliconcarbide whiskers have the following properties set out in Table IIIbelow.

TABLE III Typical Properties of As-Received Silicon Carbide WhiskersProperties Values Average Whisker Length 8-15 (micrometers) AverageWhisker Diameter 1.2-2.0 (micrometers) Average Whisker Aspect Ratio 6-10Whisker Content (weight ≧80% percent) Particulate Size (mat) [d₅₀] <20μm Whisker Surface Area (BET) >3 [m²/gram] Free Carbon Content (weight0.3 wt. % (maximum) percent) Free Silica Content (weight 4.0 wt %(maximum) percent) Oxygen Content (weight 4.0 wt % (maximum) percent)Extractable Calcium Ions 100 ppm Maximum Extractable Aluminum Ions 200ppm MaximumAs mentioned above, these as-received coarse silicon carbide whiskershave not been satisfactorily used as reinforcement in a ceramic matrix,especially for applications as a ceramic cutting tool.

Applicants have invented a method by which these as-received coarsesilicon carbide whiskers are processed so as to be suitable for use as areinforcement in a ceramic matrix. Referring to the process, thisprocess first comprises a heat treatment to reduce the oxygen content inthe form of silica (SiO₂) of the whiskers and to reduce the surfaceroughness of the whiskers wherein the surface roughness is determined bymeasuring the surface area (BET). The resultant product of this firststep is a heat-treated coarse silicon carbide whisker.

Second, the process comprises the low energy ball milling of theheat-treated coarse silicon carbide whiskers to break up the mat (i.e.,silicon carbide whiskers bonded together by silica) whereby the mat iseither eliminated or reduced without reducing the aspect ratio of thewhiskers. The resultant product of the ball milling is an as-processedcoarse silicon carbide whisker that has the preferred properties setforth below in Table IV.

TABLE IV Preferred Properties of As-Processed Silicon Carbide WhiskersProperties Values Mean Whisker Length 8-15 (micrometers) Mean WhiskerDiameter 1.2-2.0 (micrometers) Mean Whisker Aspect Ratio 6-10 WhiskerContent (weight ≧80% percent) Particulate Size (mat) [d₅₀] <10micrometers Whisker Surface Area (BET) 1-2 [mm²/gram] Oxygen Content(weight <2% percent)It should be appreciated that applicants contemplate a broader range forthe average aspect ratio wherein this ranges is between about 3 to about50. A more preferred range for the average aspect ratio is between about3 to about 10. A most preferred range for the average aspect ratio isbetween about 6 to about 8. The determination of the particulate size inthe as-processed silicon carbide whiskers was based upon visualobservation at a magnification of 500-1500× wherein there was nodistinguished mat (i.e., no mat with a dimension (or size) greater than10 micrometers).

In regard to the process of making the composite, the starting ceramicpowders and as-processed coarse silicon carbide whiskers are blendedtogether. The blend is then hot-pressed into the ceramic substrate. Thehot pressing occurs in a vacuum or protective atmosphere at atemperature of between about 1400 degrees Centigrade and about 2000degrees Centigrade at a pressure of between about 10 MPa and about 100MPa and for a duration of between about 15 minutes and 120 minutes.

Referring to FIG. 1 and the geometry of a specific embodiment of aceramic cutting tool, there is illustrated a cutting tool generallydesignated as 10. Cutting tool 10 has a rake face 12 and a flank face14. The rake face and the flank face intersect to form a cutting edge16. The specific configuration is a SNGN-453T style of cutting insertwith a T land according to the American National Standard for CuttingTools-Indexable Inserts-Identification System, ANSI B212.4-1986 (cuttingedge preparation: 0.002-0.004 inch×20 degrees chamfer). Other styles ofcutting inserts and edge preparations are acceptable and arecontemplated by the inventors to be within the scope of this invention.

As shown hereinafter, specific embodiments of the ceramic cutting toolscomprise excellent cutting tools. In machining applications whereabrasive wear resistance is more of a concern than chemical wearresistance, a titanium carbide-based matrix is preferable. If, however,chemical wear resistance is more important than abrasive wearresistance, then a hafnium carbide-based or titanium carbonitride-basedmatrix is preferable. Chemical wear resistance may also be improved byapplying a hard coating to the cutting tool substrate. Exemplarycoatings include (without limitation) titanium nitride, titaniumcarbonitride, titanium aluminum nitride, titanium carbide, and alumina.

For machining of nickel base super alloys, or for any workpiece in whicha combination of high hardness and high chemical inertness is desired,it is more preferred that a titanium carbonitride (TiC_(x)N_(y)) basedmatrix be used in which x is greater that 0 but less than 0.95 andy+x=1. More preferably, y is greater than or equal to 0.5. For x=0, thatis titanium nitride alone, the hardness may be reduced and there may bea reaction between the titanium nitride and the SiC whiskers during thehigh temperature fabrication of these materials. Therefore, y should beless than 0.95. Another range for x and y for the titanium carbonitride(TiC_(x)N_(y)) is that y is less than 0.90 and greater than or equal to0.55, and x+y=1. Still another range for x and y for titaniumcarbonitride (TiC_(x)N_(y)) is that y is less than 0.75 and greater thanor equal to 0.6, and x+y=1. One preferred composition for the titaniumcarbonitride is where x=0.5 and y=0.5. Examples of this composition arecompositions TK5 and TK6 set out in Table II hereinabove. Optionally,titanium carbonitride may be replaced in whole or in part by hafniumcarbonitride or zirconium carbonitride.

In order for the coarse silicon carbide whiskers to be suitable for useas reinforcement in the matrix they must first be processed according tothe two step method described above. This method is now described inmore detail below in connection with specific examples.

The first step is to take the as-received coarse silicon carbidewhiskers and subject them a heat treatment. The purpose of this heattreat is to reduce the amount of oxygen (in the form of silica) on thesurface of the whiskers. In this regard, the preferred oxygen content isless than about 1 weight percent. A more preferred oxygen content isbelow about 0.5 weight percent.

Another purpose of the heat treatment is to reduce the surface roughnessof the whiskers as measured by surface area (BET). The preferred surfacearea (BET) is between about 1 and about 4 square meters per gram. A morepreferred surface area (BET) is between about 1 and about 2 squaremeters per gram.

The temperature of the heat treatment ranges between about 1400 degreesCentigrade and about 1900 degrees Centigrade. A more preferred heattreatment temperature range is between about 1500 degrees Centigrade andabout 1870 degrees Centigrade. When the temperature of heat treatment isbelow about 1500 degrees Centigrade the length of the heat treatment maybecome too long to practically achieve the desired result. When thetemperature of the heat treatment is above 1870 degrees Centigrade thereis more of a risk that there will be growth of the single crystalsilicon carbide whiskers into polycrystalline silicon carbide fibersthat would have a detrimental impact upon the material properties. Theduration of the heat treatment ranges between about 15 minutes and about120 minutes. A more preferred range of the duration is between about 30minutes and about 60 minutes. The heat treatment occurs in an inertprotective atmosphere (e.g. nitrogen or argon) or in a vacuum (e.g. lessthan 500 micrometers Hg).

Although applicants do not intend to be bound by any theory in regard toreducing the surface roughness, according to solid state sinteringtheory, areas on the surface of the whisker having a positive radius ofcurvature (convex) will have a relatively high vapor pressure, whereareas on the surface of the whisker having a negative radius ofcurvature (concave) will have a relatively low vapor pressure.Difference in vapor pressure as a function of surface curvature is thedriving force to transport materials from positive curvature areas tonegative curvature areas. Such a process leads to the improvement insurface roughness of the silicon carbide whiskers. No shrinkage ordensification will occur if the processing temperature is not too high.Too high a temperature will result in recrystallization of the singlecrystalline whiskers and also result in the sintering of the whiskerstogether. Thus, temperature and holding time must be carefullycontrolled in order to achieve optimized performance. Table V set forthbelow presents the results of various temperatures and durations andatmospheres on the properties of the as-received coarse silicon carbidewhiskers.

TABLE V Results of Heat Treating Coarse Silicon Carbide Whiskers atDifferent Heat Treating Parameters Oxygen Temp Duration BET Content Lot# (° C.) (minutes) Atmosphere (m²/gram) (wt %) WX0022- — — As 4.43 1.85CLW Received WX0022- 1400 45 Argon 3.18 1.79 CLW WX0022- 1900 30 Argon2.22 0.70 CLW FB- — — As 3.34 2.03 SYN05- Received TT01 FB- 1200 60Vacuum 2.35 1.71 SYN05- TT01 FB- 1500 60 Vacuum 1.92 0.96 SYN05- TT01FB- 1700 45 Vacuum 1.70 0.98 SYN05- TT01 FB- 1800 45 Vacuum 2.07 0.47SYN05- TT01 FB- 1900 15 Vacuum 1.03 0.47 SYN05- TT01As can be seen from the results set out in Table V, the oxygen contentand the surface roughness of the as-received coarse silicon carbidewhiskers decreases upon being subjected to the heat treatment. Generallyspeaking, the reduction in the surface roughness and the oxygen contentis the greatest as the temperature increases. Table VI sets forth thereduction (in percent) of the surface area (BET) and the oxygen contentdue to the heat treatment regardless of the particle whisker lot.

TABLE VI Amount of Reduction of Oxygen Content and Surface Area (BET)Due to Heat Treatment Reduction in Reduction in Oxygen Content SurfaceArea BET Heat Treatment (%) (%) 1200 29.6 15.8 1400 28.2 3.2 1500 42.552.7 1700 49.1 51.7 1800 38 77 1900 59.6 69.6 [Average of Both WhiskerLots]

Table VII below sets forth the impact of the heat treatment parameterson the fracture toughness of a titanium carbonitride-alumina-(30 volumepercent) coarse silicon carbide composite.

TABLE VII Impact of Heat Treatment on Fracture Toughness of TitaniumCarbonitride-Alumina-(30 volume percent) Coarse Silicon Carbide WhiskerComposite Temperature Time K_(IC) (E&C) Mix No. (° C.) (Minutes)Atmosphere MPa · m½ CB347-67 Not Heat — — 6.02 Treated CB347-66 1900 30Argon 6.59 CB347-80 Not Heat — — 5.95 Treated CB347-79 1900 30 Argon6.74

The technique to measure the fracture toughness (Mpam^(1/2)) is by Evans& Charles (Evans & Charles, “Fracture Toughness Determination byIndentation”, J. American Ceramic Society, Vol. 59, Nos. 7-8, pages371-372) using a 18.5 kg load. It can be seen that the heat treatment ofthe coarse silicon carbide whiskers results in an improvement in thefracture toughness of the composite. In averaging the improvement of thefracture toughness for the above examples, the heat treatment resultedin an improvement in the fracture toughness of about 10.4 percent ascompared to a composite using as-received coarse silicon carbidewhiskers. The ceramic composite was consolidated via hot pressing at atemperature of about 1875 degrees Centigrade, a pressure of about 35MPa, a duration of about 60 minutes and in a vacuum.

Referring to the examples set out in Table VIII after completion of theheat treatment, the heat-treated coarse silicon carbide whiskers werenext subjected to a low energy ball milling treatment. The purpose ofthe low energy ball milling was to reduce the silicon carbide matcontent. The duration of the ball milling must be sufficient so as toreduce or eliminate the mat, but not so long as to damage the whiskers.The ball milling may last from between about 10 minutes to about 60minutes. The more preferred range is between about 15 minutes and 45minutes. The most preferred range is between about 30 minutes and about40 minutes.

Table VIII below sets forth the impact of the low energy ball milling onthe fracture toughness of the titanium carbonitride-alumina-coarsesilicon carbide whisker composite. For each of the examples in TableVIII, the coarse silicon carbide whisker content was 30 volume percent.The ceramic composite was consolidated via hot pressing. The hotpressing parameters were as follows: a temperature of about 1875 degreesCentigrade at a pressure about 35 MPa for a duration of about 60 minutesunder an atmosphere of vacuum.

TABLE VIII Impact of Low Energy Ball Milling on Fracture Toughness ofTitanium Carbonitride-Alumina-(30 Volume Percent) Coarse Silicon WhiskerComposite Ball Milling Time K_(IC)(E&C) Mix No. (minutes) MPa · m½CB347-27  0 5.74 CB347-63 15 6.05 CB347-54 30 7.45 CB347-55 40 7.18CB347-56 60 6.97 CB347-57 90 6.59As can be seen from the results in Table VIII, a low energy ball millingtime of 30 minutes appears to result in the greatest increase infracture toughness for the titanium carbonitride-alumina-30 volumepercent coarse silicon carbide whisker composite.

Referring to the photomicrographs, FIGS. 2 and 3 show the microstructureof the titanium carbonitride alumina composites reinforced with theas-processed coarse silicon carbide whiskers. These photomicrographsshow that the coarse silicon carbide whiskers are homogenouslydistributed on the titanium carbonitride-alumina matrix and no visualsilicon carbide whisker mat can be seen.

FIGS. 4 and 5 show the microstructure for a titanium carbonitride-basedceramic substrate containing alumina and as-received coarse siliconcarbide whiskers. These photomicrographs show the clusters of thesilicon carbide whiskers and the porosity that is stemmed from thesilicon carbide whisker mat. It is the silicon carbide whisker mat (andporosity) that result in a low sintered density, a lower hardness, and areduced fracture toughness.

FIG. 6 shows the microstructure for a titanium carbonitride-aluminacomposite reinforced with fine diameter silicon carbide whiskers. Thesilicon carbide whiskers in this composite are much finer than those inFIGS. 2 and 3.

FIG. 7 shows the microstructure of the WG-300 material made by GreenleafCorporation of Saegertown, Pa. As best understood, this material has acomposition of about 65 to about 70 volume percent alumina and about 25to about 30 volume percent silicon carbide whiskers. The silicon carbidewhiskers are supplied from Advanced Composite Materials Corporation andhave an average diameter of about 0.6 micrometers, an average length ofbetween about 30 and about 60 micrometers, and an aspect ratio of 75.This photomicrograph shows differences in microstructure. In FIGS. 2 and3, the matrix is titanium carbonitride-alumina and the microstructure ismuch coarser, the reinforced silicon carbide whiskers are much coarser(compared with FIG. 7). While in FIG. 7, the matrix is alumina and themicrostructure is very fine, the reinforced silicon carbide whiskers aremuch finer.

The silicon carbide whisker density of the titanium carbonitride (38.12volume percent)-alumina (36.63 volume percent)-silicon carbide whiskers(25 volume percent) composites like FIG. 3 and of FIG. 6 was measured inthe following way. The silicon carbide whiskers in each one of 17 imagesof a size 66.26 micrometers×49.70 micrometers (3.2929×10⁻³ mm²) wasmanually counted so as to determine a mean count and a standard error.Each image was viewed optically at a magnification of 1500× looking at aprepared surface in a plane perpendicular to the pressing axis. For thecomposite like FIG. 3, the mean count was 99 whiskers with a standarderror equal to 4.0. This equated to a silicon carbide whisker density of30,065 whiskers per square millimeter or 1202 whiskers per squaremillimeter per one volume percent of whiskers. For the composite likeFIG. 6, the mean count was 183 whiskers with a standard error equal to6.0. This equated to a silicon carbide whisker density of 55,574whiskers per square millimeter or 2223 whiskers per square millimeterper one volume percent of whiskers. The toughness of these compositeswas measured using the Evans & Charles technique. Table IX below setsout the silicon carbide whisker density in the number of whiskers persquare millimeter, the toughness (K_(IC)) in MPam^(1/2) and the densityof the ceramic composite as a percentage of theoretical density of 4.226grams/cubic centimeter.

TABLE IX Selected Properties of Titanium Carbonitride-Alumina- SiliconCarbide (25 volume percent) Composites Like FIG. 3 and FIG. 6 FractureToughness Density SiC SiCw Density (K_(IC)) (% of Composite Whiskers(Whiskers/mm²) [MPam^(1/2)] Theoretical) FIG. 3 Coarse 30,065 7 ≧99.5%FIG. 6 Fine 55,574 7.5 ≧99.5%As it is apparent from the results in Table IX above, the number of finesilicon carbide whiskers per unit area in a titaniumcarbonitride-alumina-silicon carbide whisker (25 volume percent)composite is about 1.85 times as many as the number of coarse siliconcarbide whiskers in a composite with the same composition.

In regard to the silicon carbide whisker density, applicants contemplatethat the silicon carbide whisker density as measured in whiskers persquare millimeter is equal to or less than an about 1500 times thevolume percent of silicon carbide whiskers. Another silicon carbidewhisker density in silicon carbide whiskers per square millimeter isequal to or less than about 1400 times the volume percent of siliconcarbide whiskers.

Applicants also contemplate that the silicon carbide whisker densitywould be approximately proportional to the volume percent of siliconcarbide whiskers. In this regard, FIG. 8 is a graph that shows therelationship between the volume percent of silicon carbide whiskers inthe ceramic composite (the horizontal axis) and the density of thesilicon carbide as measured in silicon carbide whiskers per squaremillimeter (the vertical axis). FIG. 8 shows Points A and B. Point Arepresents a composite with 1 volume percent of silicon carbide whiskersand a density of 1500 silicon carbide whiskers per square millimeter.Point B represents a composite with 50 volume percent silicon carbidewhiskers and a density of 75,000 silicon carbide whiskers per squaremillimeter. For a ceramic composite that has a silicon carbide whiskercontent between 1 volume percent and 50 volume percent, the preferredsilicon carbide whisker density (silicon carbide whiskers per squaremillimeter) is equal to or less than the density value as represented bythe line A-B of FIG. 8.

Applicants also contemplate that the silicon carbide whisker density isbetween about 48 percent and about 60 percent of the silicon carbidewhisker density of a comparative ceramic composite having the sameceramic matrix and same silicon carbide whisker content of fine siliconcarbide whiskers wherein the fine silicon carbide whiskers have thefollowing properties: an average diameter of about 0.6 micrometers, andan average length between about 30 and about 60 micrometers and anaverage aspect ratio of about 75.

Metalcutting tests were conducted to determine the performance ofcutting tools made from the composites using the as-processed coarsesilicon carbide whiskers. The compositions that were tested were TK5 andTK6 wherein the compositions are set out in Table II hereof. Theseexamples had the as-received coarse silicon carbide whiskers processedas follows: a heat treatment in vacuum at a temperature of about 1500degrees Centigrade for a duration of about 60 minutes. The heat treatedsilicon carbide whiskers were then ball milled for about 30 minutes.

In regard to the method of preparation for the examples, the alumina andyttria components of the ceramic matrix were blended in a mill withisopropanol for two hours. The titanium carbonitride was added and themixture was blended for another two hours. The as-processed (i.e., heattreated and ball milled) silicon carbide whiskers were separatelysonicated for two hours and then added to the blend of the othercomponents. The silicon carbide whiskers were then blended with theblend of the balance of the components in a mill for 20 minutes. Thisblend was then discharged through a 40 mesh screen, dried in a rotaryevaporator, and then passed through a 40 mesh screen. The dried blendwas uniaxially hot pressed at a temperature of about 1850 degreesCentigrade for a duration of about 60 minutes at a pressure of 35 MPaunder vacuum. The resulting product was ground into a particular styleof cutting tool.

Referring to the results of the metalcutting tests, the cuttingparameters had the following ranges: a speed between about 470 surfacefeet per minute [143 surface meters per minute] and about 1000 sfm [305sm/m]; a feed between about 0.005 inches [0.127 millimeters] and 0.009inches [0.23 mm] and a depth of cut ranging between about 0.05 inches toabout 0.2 inches (1.27 mm to 5.08 mm). The workpiece materials includediron-based, nickel-based and cobalt-based superalloys such as, forexample, HS188, Inconel 718, Inconel 625 and A286. The overallperformance of the cutting tools of the invention (TK5 and TK6) wasacceptable.

By using the coarser as-processed silicon carbide whiskers in theceramic composite, applicants believe that they have developed a ceramiccomposite that has desired properties that are similar to the ceramiccomposites that use the finer silicon carbide whiskers, but with theadded advantage of the potential to lower fabrication costs.

For example, the use of coarser silicon carbide whiskers, which have alarger average diameter (e.g., greater than or equal to about 1.2micrometers) than the finer silicon carbide whiskers (e.g., an averagediameter equal to about 0.6 micrometers), should increase the fracturetoughness of the ceramic composite given the same silicon carbidewhisker content. This would be expected to be the case because it takesa greater amount of force to break a larger diameter silicon carbidewhisker than to break a smaller diameter silicon carbide whisker. Inthis regard, please see Becher et al., “Influence of Reinforcementcontent and diameter on the R-curve response in SiC-whisker-reinforcedalumina”, J. Am. Ceram. Soc., 79[2] pages 298-304 (1996), Becher et al.,“Toughening behavior in whisker-reinforced ceramic matrix composites”,J. Am. Ceram. Soc., 71[12] pages 1050-61 (1988); and Akatsu et al.,“Effects of whisker content and dimensions on the R-curve behavior of analumina matrix composite reinforced with silicon carbide whiskers”, J.Mater. Res. 16[7] pages 1919-27 (2001).

The use of coarser as-processed silicon carbide whiskers should alsoresult in a reduction of the total whisker surface area as compared tothe use of finer silicon carbide whiskers. A reduction in the totalsurface area of the whiskers should result in a reduction of the surfaceoxygen content of the whiskers so that there should be better whiskerpull-out in the ceramic composite. Better whisker pull-out results inimproved fracture toughness for the ceramic composite.

In view of the expectation of better whisker pull-out through the use ofcoarse silicon carbide whiskers, fracture toughness values similar tothose achieved with finer silicon carbide whiskers having an averagediameter of 0.6 micrometers should be achieved at lower volume percentcoarse silicon carbide whisker loadings. By using a lower volume percentof silicon carbide whiskers, there should be achieved improved chemicalwear resistant in metalcutting applications due to the overall lowercontent of silicon carbide in the composite.

The lower density of silicon carbide whiskers per unit volume shouldalso mean that higher volume percentages of silicon carbide whiskers canbe used in ceramic composites with less of a chance of non-homogeneouswhisker distribution occurring which adversely affects toughness.Applicants believe that tougher ceramic composite wear parts should bepossible through the use of the coarse silicon carbide whiskers.

By using coarser silicon carbide whiskers in the ceramic composite,applicants expect that the process of producing the ceramic compositewill be improved. In this regard, the use of coarser silicon carbidewhiskers in the ceramic powder blend results in a lower population ofsilicon carbide whiskers per unit volume in the starting powder blend ascompared to a starting powder blend that uses finer silicon carbidewhiskers. This lower whisker population should allow for the use ofvacuum sintering followed by hot isostatic pressing (withoutencapsulation), as opposed to hot pressing, to densify the ceramiccomposite to a useful density (i.e., greater than 98 percent of thetheoretical density). What this means is that by the use of coarsersilicon carbide whiskers in the powder blend, one should be able tovacuum sinter and HIP a powder blend that has a higher content ofsilicon carbide whiskers (i.e., greater than about 10 volume percentsilicon carbide whiskers, and more preferably, equal to or greater thanabout 15 volume percent silicon carbide whiskers) to a useful densitythan if finer silicon carbide whiskers were used in the blend). SeeTiegs, U.S. Pat. No. 4,652,413. This greatly decreases the fabricationcosts of the ceramic composite. In addition, it would be expected that alower whisker population would result in a reduction of the hot pressingtemperature to achieve a useful density.

The use of coarser silicon carbide whiskers in the starting powder blendshould provide for better flow of the starting powder blend as comparedto a blend that uses the finer silicon carbide whiskers. Better powderflow should result in better and easier processing of the powder blend.The use of the coarser silicon carbide whiskers also should result in areduction of the potential for a non-homogeneous distribution of thesilicon carbide whiskers in the ceramic composite.

The use of coarser silicon carbide whiskers also should result in lowerproduction costs as compared to a ceramic composite that uses finersilicon carbide whiskers. This is true because coarser silicon carbidewhiskers are typically less expensive to purchase than finer siliconcarbide whiskers.

Overall, by the use of coarser silicon carbide whiskers as compared tofiner silicon carbide whiskers applicants should be able to produce aceramic composite that has better properties (e.g., fracture toughness).Applicants should be able to make the process to produce the compositeeasier and less costly through improved powder flow and by the abilityto use vacuum sintering and HIP techniques to densify the composite.Applicants should be able to reduce the starting material costs throughthe use of coarser silicon carbide whiskers.

All patents, patent applications, articles and other documentsidentified herein are hereby incorporated by reference herein. Otherembodiments of the invention may be apparent to those skilled in the artfrom a consideration of the specification or the practice of theinvention disclosed herein. It is intended that the specification andany examples set forth herein be considered as illustrative only, withthe true spirit and scope of the invention being indicated by thefollowing claims.

1. A method of treating as-received silicon carbide whiskers having thefollowing properties: an oxygen content of about 3 to about 5 weightpercent, a surface area of greater than about 3 square meters per gram,comprising the steps of: heat treating the as-received silicon carbidewhiskers at a temperature between about 1400 degrees Centigrade andabout 1900 degrees Centigrade for a duration between about 15 minutesand about 60 minutes to form heat treated silicon carbide whiskershaving the following properties: an oxygen content of less than about 2weight percent of the silicon carbide whiskers, a surface area belowabout 2 square meters per gram.
 2. The method according to claim 1wherein the heat treated silicon carbide whiskers are subjected to ballmilling for a duration between about 10 minutes and about 60 minutes soas to produce as-processed silicon carbide whiskers.
 3. The methodaccording to claim 2 wherein the average aspect ratio of theas-processed silicon carbide whiskers is about the same as the averageaspect ratio of the heat-treated silicon carbide whiskers.
 4. The methodaccording to claim 2 wherein the as-received silicon carbide whiskershaving up to about 20 weight percent of silicon carbide clusters havinga size greater than about 10 micrometers and the content of the siliconcarbide clusters is reduced as a result of the ball milling step.
 5. Themethod according to claim 4 wherein there is an absence of the siliconcarbide clusters in the as-processed silicon carbide whiskers.
 6. Themethod according to claim 1 wherein as a result of the heat treatingstep the oxygen content of the heat-treated silicon carbide whiskers isbetween about 25 percent and about 65 percent less than the oxygencontent of the as-received silicon carbide whiskers.
 7. The methodaccording to claim 1 wherein as a result of the heat treating step thesurface area (BET) of the heat-treated silicon carbide whiskers isbetween about 28 percent and about 60 percent less than the surface area(BET) of the as-received silicon carbide whiskers.